Conventionally proposed boiler systems for combusting a plurality of boilers to generate steam include a boiler system of the so-called proportional control type, for continuously increasing or decreasing a boiler combustion amount to control a steam flow (e.g. Patent Document 1). Such a boiler system of the proportional control type can finely regulate the generated steam flow and improve pressure stability.
A boiler system typically secures, as reserve power, a steam flow approximately corresponding to a sudden load variation or temporary increase of a necessary steam flow. Reserve power can be secured most easily by increasing the number of combusted boilers.